lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 20

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

LT3686A
APPLICATIONS INFORMATION
Short and Reverse Protection
If the inductor is chosen so that it won’t saturate excessively,
the LT3686A will tolerate a shorted output. When operat-
ing in short-circuit condition, the LT3686A will reduce
its frequency until the valley current is 1.7A (Figure 13).
There is another situation to consider in systems where
the output will be held high when the input to the LT3686A
is absent. This may occur in battery charging applications
or in battery backup systems where a battery or some
other supply is diode OR-ed with the LT3686A’s output.
If the V
held high (either by a logic signal or because it is tied to
V
quiescent current through its SW pin. This is fi ne if your
system can tolerate a few mA in this state. If you ground
the EN/UVLO pin, the SW pin current will drop to essentially
zero. However, if the V
is held high, then parasitic diodes inside the LT3686A can
20
IN
Figure 13. The LT3686A Reduces its Frequency from
2MHz to 160kHz to Protect Against Shorted Output
), then the LT3686A’s internal circuitry will pull its
500mA/DIV
20V/DIV
IN
V
SW
pin is allowed to fl oat and the EN/UVLO pin is
I
L
V
L = 6.8μH
C
R
V
IN
OUT
OUT
T
= 17.4k
= 35V
= 22μF
= 0V
IN
pin is grounded while the output
2μs/DIV
3686A F13
pull large currents from the output through the SW pin
and the V
only when the input voltage is present and that protects
against a shorted or reversed input.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3686A circuits. However, these
capacitors can cause problems if the LT3686A is plugged
into a live supply (see Linear Technology Application
Note 88 for a complete discussion). The low loss ceramic
capacitor combined with stray inductance in series with
the power source forms an underdamped tank circuit,
and the voltage at the V
to twice the nominal input voltage, possibly exceeding
the LT3686A’s rating and damaging the part. If the input
supply is poorly controlled or the user will be plugging
V
IN
Figure 14. Input Diode Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output; it Also
Protects the Circuit from a Reversed Input. The LT3686A
Runs Only When the Input is Present
IN
pin. Figure 14 shows a circuit that will run
EN/UVLO
SYNC/MODE
V
SS
RT
IN
LT3686A
GND
IN
pin of the LT3686A can ring
BOOST
SW
BD
DA
FB
3686A F14
3686af
V
OUT